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Application of PCAN interface card based on Hongke in hemodialysis machine
The number of patients with end-stage renal disease in China is huge and increasing year by year. Hemodialysis is a relatively mature alternative therapy, which can alleviate patients' symptoms and prolong patients' life. At present, China's hemodialysis machine market is mainly occupied by imported brands. The research and development of domestic hemodialysis machines with high quality and low price is not only the pursuit of China's medical enterprises, but also of great significance to the progress of science and technology and the development of medical undertakings.
Hemodialysis is to drain the blood in the body to the outside of the body through a dialyzer composed of countless hollow fibers. The blood and electrolyte solution (dialysate) with similar body concentration are inside and outside the hollow fibers, and exchange substances through dispersion / convection to remove metabolic waste in the body, maintain electrolyte and acid-base balance, and remove excessive water in the body.
Challenge
Shortcomings of traditional hemodialysis machine
In the actual treatment in the hospital, different modules need to be selected for treatment according to the situation of patients. After years of development, the function, safety performance and humanized design of domestic advanced models are comparable to those of foreign models, but domestic hemodialysis machines still have shortcomings. For example, most hemodialysis equipment does not have the functions of blood volume monitor, multiple sets of independent CPU systems and central networking. Although it has a variety of safety monitoring functions, the actual failure rate is high. Whether it is the appearance of the equipment, the principle of the hydraulic system of the machine, the design of parts, etc., there is a lack of innovation and lack of core competitiveness.
programme
Application and optimization of PCAN interface card based on Hongke in hemodialysis machine
In order to optimize the original prototype according to the new requirements and higher requirements, it is necessary to consider the system integrity, safety, accuracy and so on. Based on the above problems, Hongke PCA interface can be used to further R & D and optimization of hemodialysis machine.
Hemodialysis machine has many electronic control parts, which has high requirements for various indicators in the work of the machine. It needs an efficient and stable automatic control system to make dialysis run safely. Therefore, the control system of hemodialysis machine should meet the basic requirements of automatic control system, that is, stability, rapidity and accuracy.
For the control system of hemodialysis machine, its safety includes not only the control system itself, but also the executive parts controlled by the control system. When the control system has its own security, it should know the operation of the executive parts. When the actuator fails or does not work normally occasionally, the control system should be able to infer what parts have failed, what faults have occurred, what causes the faults and how to deal with them from the feedback of its components or according to the overall operation of the system.
Therefore, the functions of the control system structure of hemodialysis machine include: collecting the signals of various sensors to ensure the safety of dialysis; Control the movement of each pump to achieve accurate control of important parameters and meet the standard.
The control system of Hongke scheme can be divided into upper layer and lower layer. The upper layer is the main controller of upper computer and lower computer, and the lower layer is the motor and its driver, sensor module and three solenoid valve control boards. Its hardware structure is shown in the figure. The control system mainly includes the upper controller, the lower executive components and sensor modules. The upper computer is responsible for human-computer interaction function, obtaining medical staff instructions, displaying treatment parameters, etc. the main controller of the lower computer receives the instructions of the upper computer, completes the functions of the hemodialysis machine, and uploads the parameters to the upper computer. The motor, driver and solenoid valve are the executive components, The sensor acts as an information feedback component.
According to the requirements of the control system, the functions of each hardware module are divided as follows:
1) The host computer mainly realizes the human-computer interaction function, including guiding medical staff to carry out dialysis operation, displaying dialysis parameters in real time, providing maintenance personnel with maintenance platform, etc. The upper computer communicates with the main controller of the lower computer through CAN bus.
2) The main controller of the lower computer is used to realize the overall control of the hemodialysis machine, that is, collect sensor information such as temperature, conductivity and flow, control motors such as blood pump, degassing pump and flow pump, and control the drive of solenoid valve controller.
3) The motor and driver are the actuator of the control system, and the sensor module is the information acquisition device.
4) The solenoid valve controller is responsible for driving the solenoid valve and feeding back the status of the solenoid valve.
In this communication process, the can communication task is mainly responsible for the communication of devices on the CAN bus. We can give the highest priority to communication tasks to ensure that the control commands and parameters of the upper computer can be transmitted to the lower computer in time at any time. All communication between the upper computer and the lower computer is completed by this task, and other tasks can only contact the upper computer indirectly through this task. In the can receiving interrupt, it receives the data sent by the upper computer, caches the data, and triggers the can communication task by sending semaphore for analysis and execution.
HONGKE
Key characteristics of Hongke PCA interface card
high speed USB 2.0 adapter (compatible with USB 1.1 and USB 3.0)
two D-sub connectors are used to send and receive can FD and Lin messages (both are allocated by pins of can FD and LIN bus)
comply with can specification 2.0 A / B and FD
each can FD channel shall separate USB and Lin, with a maximum of 500V
can terminal can be excited by welding jumper
bus load measurement includes error frame and overload frame
induce error to generate can message for input and output
summary
The application and optimization of Hongke PCAN interface card in hemodialysis machine can meet the requirements of product stability, rapidity and accuracy. It can also be used with our handheld tester PCAN diag FD for various fault diagnosis, such as various fault diagnosis by processing CAN2.0 and can FD messages in the protocol layer, and oscilloscope function and voltage in the physical layer Resistance measurement function for fault diagnosis. At present, many manufacturers at home and abroad have used PCA interface card to upgrade their medical equipment. If you want to know more about PCA interface card, please contact Hongke for product information and trial opportunities.
Original title: [PCA medical application series] part6 application in hemodialysis machine based on Hongke PCA interface card
The source of the article: [WeChat official account: Guangzhou Rainbow Electronics Technology Co., Ltd.] welcome to add attention!
With the development and continuous improvement of the monitoring function of medical equipment, telemedicine providers can provide better diagnostic tools for home patients, emergency room paramedics and hospitals in several important areas of human health. Sphygmomanometers, blood glucose meters, defibrillators and other monitoring instruments need clear analog signals for accurate measurement, otherwise they may be life-threatening. The excellent design of analog signal path can help designers reduce the interference of external noise, expand the dynamic range and enhance the accuracy. In addition, in terms of component selection, designers must also choose carefully to meet the performance requirements of the final product.
High performance requirements in small packages
Previously, it was generally believed that medical equipment in hospitals and clinics was more accurate than portable instruments used at home. However, new technological trends are rapidly reversing this view. The users of the new portable medical devices are not only ordinary consumers, but also patients with deep scientific and technological understanding. Therefore, the needs of customers are no longer limited to taking body temperature, ECG and blood pressure. What customers need is a full range of nursing and measurement functions.
In order to meet people's urgent demand for home medical diagnostic instruments, equipment suppliers are relying on advanced inventory management and innovative design to enhance market competitiveness, and equip products with more functions to win more users. In the field of developing home medical instruments, one factor is very important, which is the development time required from the initial design to the real launch of the product. Shortening the time to market can enable manufacturers' products to seize the market. Whether the development cycle can be shortened depends on whether the design of system designers is flexible and cost-effective.
Process technology affects system design
Although electrical specifications are the main factor for designers to select components, the process used to manufacture integrated circuits is also important. For example, a typical blood glucose meter usually needs to be equipped with an operational amplifier with very low input bias current. Most designers will choose JFET amplifier. However, they should consider the temperature before making a decision.
Because JFET has a very low initial input bias, it is vulnerable to temperature changes. For every 10 C rise, the input bias will approximately double. To calculate the drift of the input bias, use the following formula (reference 1).
Ib(T)Ib(T0) x 2(T-T0)/10
For example, a JFET Input Operational Amplifier (such as National Semiconductor's lf411) has an input bias current of 50pA at 25 C, while a better option is National Semiconductor's lmp7731, which is a bipolar input operational amplifier with an input bias current of 1.5na. Through the above formula, we can quickly calculate that the input bias current of lf411 becomes 3.2na at 85 C, which is twice that of lmp7731.
Trade off of evaluation system
Speed, noise and power consumption may be equally important for some designs. A low-noise device will consume more current, while a low-power device can only provide limited bandwidth. One way to overcome these problems is to use inverse compensation amplifiers in appropriate applications. Compared with the stable unit gain and high speed, the advantage of the anti compensation amplifier is that it can provide a large bandwidth without affecting the power consumption.
Inverse compensation operational amplifier is most suitable for current voltage conversion (transimpedance) circuit. In medical instruments, one of the most common applications is to measure the oxygen content in blood cells, which is called SpO2 or saturated or peripheral oxygen. Figure 1 shows the block diagram of SpO2 module, in which the anti compensation amplifier (TIA) is used to convert the current from the photodiode into voltage.
Figure 1 typical block diagram of SpO2 module
Use shortcuts to shorten design time
The most important parameter of medical instruments is noise, which can cause serious interference to the circuit itself and nearby equipment. Calculating noise is a tedious task, especially when you want to calculate the overall impact of signal path on signal-to-noise ratio from power supply, amplifier, data converter and external components.
Generally speaking, medical instrument circuits tend to work at a lower frequency, so the designers of these systems usually pay more attention to the noise in the frequency band of 0.1 to 10Hz, also known as peak to peak noise. Unfortunately, some data sheets do not provide time domain noise (peak to peak) values, but only typical charts of voltage or current noise density. In addition to waiting for the circuit supplier to provide measurement data, there is a fast method to help calculate the peak to peak noise.
Suppose you intend to use lmp7731 of national semiconductor to calculate the noise of peak to peak (0.1 to 10Hz) voltage. First, select a point in the frequency range within the specified frequency band, such as 1Hz, and the value in the comparison curve is 5.1nv / Hz (Fig. 2). Then use the following formula to calculate the root mean square value (RMS) of noise: Formula 1: ENRMS = "enf" ln (10 / 0.1), Where enf is the noise at 1Hz. The total root mean square noise of 10.9nv can be obtained through the above formula. If you want to calculate the peak to peak noise, you only need to multiply this root mean square value by 6.6 to obtain 72.2nv. The result of this estimation is quite good, which is close to the specification 78nv listed in the data sheet.
Fig. 2 Relationship between input voltage noise and frequency of lmp7731 frequency and voltage noise
If the voltage noise density diagram in the data sheet does not represent the noise value at 1Hz, you can use the following simple equation (equation 2) to calculate the value at a certain frequency.
Where ENB is broadband noise (usually the value at 1kHz), and FCE is the 1 / F inflection point. As for F is the frequency of interest, in our case, it is 1Hz.
For example, the broadband noise of lmv851 of national semiconductor is 10nv / Hz at 10kHz. In order to calculate the root mean square noise, first determine the value of 1 / F inflection point (FCE) from the chart. Using the voltage noise density diagram in the data sheet, it can be found that FCE is approximately equal to 300Hz. Then, using the above formula, we can calculate en = 10 * (300 / 1) = 173nv Hz, which is the voltage noise at 1Hz. Finally, substitute this value into Formula 1 and multiply the result by 6.6 to get 2.4 Peak to peak noise of V.
Another consideration is current noise. Generally speaking, if the impedance of the power supply is not very large (> 100k ), you can still get a very close calculation result without considering the current noise, just like the above example. However, if the impedance of the power supply is large, the same technique must be used to calculate the current noise, and the voltage and current noise must be added in the form of root mean square value.
Requirements for determining speed
Just as the noise of operational amplifier is very important to the resolution of ADC, so is the bandwidth to maintain the accuracy of the system. In order to limit the error to 1 / 2 least significant bit (LSB), a fast check is needed to determine whether the bandwidth of the amplifier is sufficient. In addition to using complex and generic guidance, you can also use the resolution of the analog / digital converter to quickly calculate the results. The method is to use 1 / 2 (n / 2) and multiply the result by the amplifier frequency at - 3dB (reference 2).
Through the above shortcut and a 14 bit ADC, FEFF = 0.007813 * f-3db is obtained in this example. For the operational amplifier (LMP7711) with configurable gain of 10 in Fig. 3, the frequency at - 3dB is 1.7MHz. In this way, the maximum bandwidth (at 1 / 2 LSB error) is equal to 0.007813 * 1.7e6 = 13.3khz.
Fig. 3 block diagram of portable electrocardiograph
Monitor device and communication device
Most of the newer medical diagnostic instruments are equipped with wireless communication function. Modern electrocardiograph (EKG or ECG) can transmit patient data to doctor's clinic or hospital in a few minutes through personal electronic manual account (PDA) or other computer peripherals. Despite the benefits of wireless data transmission, such devices may cause serious interference to medical devices and make their readings wrong.
In order to avoid this interference, a filter must be used. However, adding filters will not only increase the volume of the equipment, but also increase the design cost. A more cost-effective and fast way is to use components (including filters) that can suppress radio frequency (RF) noise.
epilogue
Nowadays, the trend in the field of medical equipment is to provide consumers with higher value products, that is, home care instruments that are cheap and high-quality and can quickly provide diagnostic results. With the continuous progress of technology, more and more medical devices will transmit data from patients' homes to doctors' clinics in real time through computers. In addition, with the demand of users for more functions, the accuracy requirements for portable medical devices will be further improved to achieve more accurate diagnosis, which will rely on the continuous innovation, long-term development and commitment to comprehensive solutions made by designers.
Power supply standardIEC 60601-1:2005 and accompanying documents specify the safety and basic functional requirements of the power supply. The standard appears in the form of ANSI / AAMI document in the United States and in the form of "euronorm" in Europe and around the world, with some local differences. Since January 2021, the latest revision (a1:2012, a2:2020) has updated the standard, deleted outdated references, corrected errors and made it better consistent with IEC 62368-1. The current medical EMC standard IEC 60601-2 is the 4th Edition.Built in medical power standard
AC powered medical devices can have "built-in" power supplies or lower power external adapters. If it is an internal product, the product designer should consider high voltage and high energy, and usually choose to buy a "medically certified" power supply. However, the final product is the product certified for this application, so there are other considerations, such as labels, connectors, internal wiring of power supply and fuses.
Due to grounding, external wiring and interference from other noise sources, the built-in power supply conforming to EMC standards cannot be guaranteed to pass the test. A common practice is to add another EMI filter at the AC inlet, which may be included in the connector. However, it should be noted that the leakage current limit should not be exceeded. For the most stringent CF product rating, the leakage current limit can be as low as 10 a. The leakage of external filter may directly lead to the internal leakage of AC power supply, there is a risk of interaction between external filter and internal filter, and may even worsen EMI.
Secure power solutionsThe safe solution should select the target application and specify a power supply with 2 x MOPP certification and EMC compatible performance. For example, power supplies from recom racm230-g, racm550-g and the recently released racm-1200v series.Their peak or rated power is 230W, 550W and 1200W respectively. When fans are not allowed (usually in medical environments), these products can be floor cooled. All products have 2x MOPP / 250V AC rating to prevent leakage to ground (racm550 / 1200). The safety barrier in each recom power supply is shown in Figure 2.
Figure 2: safety fence racm230 / 550 / 1200 seriesFigure 3: a 2x MOPP DC / DC converter provides SIP / SOP isolationUsing a medically certified power supply (such as the power supply of recom) is a reliable way to ensure that the safety and EMC standards required by various medical application environments are met. It would be a lower overall cost approach if development, certification costs and time to market were taken into account.
Recom is a good choice when selecting AC / DC power supply for medical equipment. Eurotime electronics, as a distribution partner of recom, provides local inventory and technical support.Original title: eurotime classroom | AC / DC power supply in medical applicationsThe source of the article: WeChat official account: Euro time electronic RS] welcome to add attention! Please indicate the source of the article.
Capacitive detection technology continues to be favored in traditional man-machine interface applications, such as laptop touch panel, MP3 player, touch screen display and short-range detector. In addition to using capacitive sensors to replace mechanical buttons, a little imagination and the basic principle of man-machine interface design will enable many other applications to use this technology. Figure 1 shows examples of application concepts that can be improved by using human contact detection.Figure 1. Device using capacitive sensor electrodes
For the device shown in Fig. 1, it is often advantageous to know the contact quality between the device and the skin before starting the device or measuring. These devices include medical probes that need to be close to the skin, biopotential electrode sensors, or housings for fixing catheters. In order to determine the contact conditions, several green capacitive sensor electrodes in the figure can be directly embedded into the plastic shell of the device during injection molding production. The host microcontroller reads some status registers on the capacitive sensor controller IC, which indicate how close the capacitive sensor is to the skin. Then, the basic detection algorithm running on the host microcontroller processes the status register information to determine whether the contact between each sensor electrode and the skin is appropriate.
In the traditional man-machine interface application of capacitive detection, people usually touch the sensor electrode by finger touch. The example in Fig. 1 uses a capacitive sensor in an unconventional way, and the user places a device containing a capacitive detection electrode on the human body. Developing such applications is simple, but in order to build a stable and reliable system, some key guidelines should be followed
Capacitance digital controller. To develop high-performance contact detection applications, we must first select a suitable capacitor digital controller (CDC). For the application shown in Fig. 1, the contact between the device surface and the skin is directly measured by a slight change in energy, which is distributed in the capacitive sensor electrode array and occurs when the device contacts the skin. The accuracy of this measurement depends on the sensitivity of the CDC analog front end and the number of sensor electrodes. The accuracy of capacitive sensor manufactured by traditional PCB process is usually in the range of 50 FF to 20 PF, so the high-precision measurement technology using 16 bit CDC is ideal.
When selecting CDC, we should first clarify some key characteristics, such as high-resolution analog front-end with 16 bit ADC, programmable sensor sensitivity setting, programmable sensor offset control, on-chip environment calibration, sufficient capacitive input channels supporting an ideal number of sensor electrodes, and integrated design without using external RC devices for sensor calibration. These features support reliable and flexible applications and bring the best user experience. For example, programmable sensitivity allows the interface designer to preset the best sensor sensitivity for a specific application rather than adopting a fixed solution that may lead to poor sensitivity. Programmable offset control is another important feature for interface designers, because the offset value of sensor boards in each production batch may be slightly different. The quick preview feature allows you to change the host firmware settings before putting a new sensor board into mass production. For applications where ambient temperature or humidity is expected to change, on-chip environmental calibration can achieve a more reliable solution. Note that the electrode sensor is constructed using standard PCB copper traces; The properties of the substrate will change with the change of temperature and humidity, so the baseline level of the sensor output will be changed. If CDC supports on-chip calibration, this baseline drift can be dynamically compensated during product use.
Small electrodes require high sensitivity. The objective of measurement is to determine the closeness of the equipment to the skin; The better the contact quality between the skin and the equipment, the more accurate the reading of the equipment. The accuracy of measurement depends on the number (the more electrodes, the higher resolution) and size of electrode sensors distributed in the contact area of the device. For the application shown in Figure 1, the surface area of the device is generally very small, and designers need to use small sensor electrodes when developing applications.
In order to reliably measure the small capacitance change related to the small sensor electrode (generally less than 50 PF), a high-sensitivity analog front-end controller is required. Remember that the type and thickness of plastic covering material will further affect the small signal emitted by the sensor through the plastic. The analog front-end measurement of the controller must have sufficient sensitivity to measure this small signal, and maintain a good signal margin between the measured signal and the threshold level detection setting under all operating conditions (such as different power supply voltage, temperature and humidity, and the thickness and type of coverage material). Low signal margin will increase the risk of false detection and sensor instability. In order to minimize risk, when using CDC with 16 bit ADC, a margin of at least 1000 LSB shall be maintained between the sensor baseline level (the sensor is not in contact with the skin) and the contact threshold level.
Ad7147 and ad7148 captouch programmable controllers are used for single electrode capacitance sensors. They have 16 bit resolution, can carry out nano farad level measurement, and can set 16 programmable threshold detection level values in the full-scale range. Both controllers support 3 mm & Tides under 1 mm plastic covering material with a dielectric constant of 3.0; 3 mm small sensor electrode while still maintaining the full-scale signal margin of 1000 ADC LSB. The full-scale signal margin is the difference between the sensor output without skin contact and with skin contact.
Maintain reliable performance. The capacitive sensor electrode is made of standard copper material or flexible material on PCB. The properties of this material change with temperature and humidity. This change will offset the baseline level (the electrical average of all sensor thresholds is based on the baseline level). A large baseline offset increases the risk that the contact threshold level is too low or too high (too low or too high depends on the direction of baseline offset), which will cause false contact error, or make the threshold level either too sensitive or not sensitive enough, resulting in the instability of contact state. In order to maintain the original signal contact threshold detection level margin (sensitivity) of the sensor, CDC needs to automatically track the amplitude of baseline offset error and readjust the threshold setting accordingly. The example in Figure 2 shows how the threshold levels of ad7147 and ad7148 are automatically tracked and adjusted for baseline offset changes caused by changes in environmental conditions.
Figure 2. Ad7147 / ad7148 on chip environment calibrationEliminate measurement errors. The modification of capacitive sensor electrode array in the device may cause space constraints and force the designer to place the CDC away from the capacitive sensor. This will increase the length of parallel sensor routing and make the wiring dense, which is not conducive to capacitive detection applications, because routing at different DC potentials will form the stray coupling path shown in Fig. 3a. The grounding layer of PCB cannot prevent this situation because the wiring and grounding layer are at different DC potentials, which will still form stray capacitance (Fig. 3b).Figure 3. The path of stray capacitance shows the results of the following parallel routing: parallel routing (a) without copper pouring layer, parallel routing (b) on grounding copper pouring layer, and parallel routing (c) on copper pouring layer with the same DC potential as the routing
To eliminate the stray capacitance error, one method is to surround the adjacent routing with a layer driven by the DC level (the DC level is the same as the DC level of the capacitive sensor electrode and routing). The ad7147 and ad7148 devices eliminate stray capacitance by providing a dedicated acschield output with this function, as shown in Figure 3C.
Consumer health care equipment such as spa and skin care products are entering ordinary families from professional institutions, and users are no longer technicians who are specially trained and familiar with products and their applications. Therefore, many of these products need a more intelligent user interface to enable untrained users to master the correct product use methods. Capacitive detection provides new choices for user interface designers, enabling them to explore various innovative methods to meet new user interface requirements. Capacitive digital technology provides contact information between capacitive sensor electrode and skin, which can be used to maintain the best product performance and safety.
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